In a TOF-MS, generally, ions accelerated by an electric field of a preset strength are thrown into a flight space where no electric field and no magnetic field is present. Since the initial speed of the ions and the time of flight in the flight space depends on the mass to charge ratio of the ions, the ions are separated by the mass to charge ratio until they are detected by an ion detector placed at the other end of the flight space. The difference in the time of flight (flight time) of two ions having different mass to charge ratios is larger as the flight path is longer. Thus, in order to enhance the resolution of a TOF-MS, it is better to obtain a longer flight path of ions. Due to the restriction to the overall length of the device, it is generally difficult to hold a long straight flight path Thus there have been proposed various types of TOF-MS that include effectively long flight paths.
In the Japanese Patent Application Publication No. H11-135060, a dual-circle closed orbit of the letter “8” is used for the flight path, and the ions run the orbit many times to attain an effectively long flight path.
There is a problem in the method, As shown in FIG. 15, in which the 8-shaped orbit of the above TOF-MS is depicted as a simplified form of a single circular orbit, ions ejected from the ion source 1 are introduced into the flight space 2 through the gate electrode 4 and are led to the circular orbit A. Note that structures and elements necessary to lead ions to the circular orbit are not shown in FIG. 15. After running on the circular orbit A one turn or more than one turn, ions leave the orbit A and the flight space 2, and are detected by the detector 3. Since the effective length of the flight path of ions becomes longer as the number of turns on the circular orbit A is increased, the difference in the flight time of ions slightly different in their mass to charge ratios becomes larger, so that they can be separated easier
The system has a drawback as follows Ions of smaller mass to charge ratios run faster on the circular orbit A, so that they can catch up to slower ions having larger mass to charge ratios after turning a plurality of times, and both ions may leave the orbit A and enter the detector 3 almost at the same time. This catch-up happens not only in such a round orbit but also in a linear reciprocal or in a curved reciprocal path.
It means that, in the above structure, ions having close mass to charge ratios can be easily separated, but ions having a large mass to charge ratio difference cannot be separated when faster ions catch up to slower ions. In order to avoid the problem, conventional TOF-MSs restricted the mass to charge ratio of ions entering through the gate electrode 4 in the circular orbit A so that such a catch-up was prevented and Ions of a large mass to charge ratio difference could not be detected at the same time.
In this case, however, when ions of a wide mass to charge ratio range were intended to be measured, the wide range had to be divided into some narrower ranges, and measurements had to be repeated for those narrower ranges. Such repetitions of measurements are of course inefficient, and are sometimes impossible when the amount of available samples is very small.